Origen

The medieval English word slur meant ‘thin, fluid mud’. Early senses of the verb were ‘to smear’ and then ‘to criticize’—you can see the same metaphor at work in the phrase ‘mud-slinging’ and in the history of the word aspersion. Later on it came to mean ‘to gloss over a fault’, and from this developed the idea of speaking indistinctly. Slurry (Late Middle English) also comes from medieval slur, and here the connection with mud is much clearer.